This invention relates generally to measuring and testing systems and methods-of use, and more specifically to systems and methods of use for testing of underground bare neutral conductor electric cable, without removing the electric cable from operation, to detect active corrosion, harmonic distortion, metal loss of the bare concentric neutrals, and grounding anomalies that are detrimental to the safe and efficient operation of the underground electric distribution system.
Since the early 1960's electric cable has been installed underground throughout the United States. This electric cable was originally installed with bare concentric electric neutrals. The purpose of the bare neutral was to provide a safe path for the flow of the cable's rated current during fault conditions resulting either from cable failure and/or third party damage. Further, the bare neutrals provide additional grounding in high resistivity soil, an equipotential environment to reduce electric stress on the cable, and shielding of the cable's electric field.
Within a few years of installation, problems began to arise from corrosion of the bare neutrals. Failures of these neutrals were reported in many earth environments and under numerous service conditions. To mitigate the corrosion of bare neutral underground cable, classical corrosion direct current potential surveys were conducted in conjunction with the subsequent installation of cathodic protection. After years of corrosion mitigation efforts, it became apparent that conventional corrosion survey techniques were ill suited for the detection of areas of corrosion and neutral loss. Conventional corrosion surveys were found to miss actual areas of corrosion while at the same time indicating corrosion where no corrosion existed.
In the mid 1980's as a result of the poor corrosion resistance of bare neutral underground electric cable, companies began to install sheathed neutrals on all new underground electric transmission and distribution electric cables. During the period prior to the adoption of sheathed electric cable, millions of miles of bare neutral cable were installed in the United States. Electric companies have been searching for a method to identify the corrosion problems on underground electric cables as well as the areas were the neutrals are damaged without removing the electric cables from service. No method to date has been effective in accomplishing these tasks economically and with a degree of reliability necessary to prevent power service interruption.
For example, in U.S. Pat. No. 4,839,598 (Calvert et al.) there is disclosed a system for detecting breaks in the bare neutral conductors of an underground cable. The system entails taking the cable out of service so that its neutral conductor and phase conductor (i.e., a main power conductor) are connected together at one end of the cable, while an alternating test signal is applied across the neutral conductor and the phase conductor at the other end of the cable. The electrical potential of the alternating signal is measured at a series of spaced apart points on the earth's surface along the cable's path. The potential between those points over a cable section with an open in the neutral conductor will be about two orders of magnitude greater than the potential between points above a good cable section. The system disclosed in the aforementioned patent is also the subject of an article entitled Overground Method Pinpoints Concentric-Neutral Corrosion, by Donald K. Baver, appearing in Transmission & Distribution, July 1989, pp.48-54.
One of the drawbacks of the prior art, like that described above, is the requirement that the electric cable be removed from service during the testing. Another drawback is that it does not establish percentage of allowable neutral loss, i.e., while the prior art method locates breaks in the neutrals, it does not determine the degree of metal loss where corrosion protection may be applied to mitigate any further corrosion damage prior to the need for cable replacement. Another drawback of the prior art is that it does not employ a concurrent direct current potential survey to correlate areas of active corrosion versus areas of other grounding abnormalities. Finally the existing method does not employ waveform analysis of the alternating current in the conductor and earth to detect 60 Hertz harmonic activity and half wave rectification which is not corrected may lead to further cable damage of either cathodically protected existing cable or replacement cable.